Due to their high efficiency and durability, light-emitting diodes (LEDs) are desirable candidates for providing general lighting in homes, offices and other environments. Whereas conventional incandescent lamps are only about 3% efficient, LEDs have efficiencies of 30% or higher. LED lifetimes are also over 20 times longer than incandescent lamps and over 5 times longer than compact fluorescent lamps.
Although the lighting performance characteristics of LEDs are superior to more conventional lighting technologies, widespread adoption of LED lighting has been slow. The primary reason for the delay is that LED bulbs are expensive. In fact, at the present time LED bulbs cost about 10-25 times more than incandescent bulbs of comparable light output.
The high price of LED bulbs is significantly impacted by the costs involved in their manufacture, in particular the costs involved in manufacturing the power conversion circuitry needed to power the LED bulbs. Incandescent bulbs receive power directly from the AC mains. However, LED bulbs are DC powered. Consequently, if power from the AC mains is to be used, an LED bulb must be equipped with power conversion circuitry to convert the AC mains power to DC power.
FIG. 1 is a drawing of a prior art LED bulb 100, illustrating how AC power from the AC mains is converted to DC in existing LED bulbs. First, a bridge rectifier (i.e., diode bridge) 102 rectifies the AC input voltage Vin from the AC mains to DC. The rectified voltage is then filtered by a smoothing circuit, which in its simplest form comprises a smoothing capacitor 104 coupled to the output of the bridge rectifier 102. Finally, a DC-DC converter 106 steps down the rectified and filtered voltage to the appropriate DC output voltage Vout needed to power the LEDs in an LED string 108. The DC output voltage Vout is set based on the number of LEDs that are in the LED string 108, the number which is determined during design depending on required light output level (i.e., lumens) of the LED light bulb 100. Physically, the LEDs of the LED string 108 are arranged in a cluster and encased by diffuser lenses, which spread the light produced by the LEDs.
One well-known problem with the power conversion circuitry of the LED bulb 100 is that the bridge rectifier 102 and smoothing capacitor 104 present a nonlinear load to the AC mains. This nonlinearity causes the input current Iin from the AC mains to be drawn in the form of a series of narrow current pulses, as illustrated in FIG. 2. The narrow current pulses are rich in harmonics of the line frequency and characteristic of a power converter having a low power factor. Power factor is a dimensionless number between 0 and 1, describing how effectively a power converter is at transferring real power from an AC power source to a load. A low power factor is highly undesirable since it results in reduced conversion efficiency, heating in the AC mains generator and distribution systems, and noise that can interfere with the performance of other equipment.
To avoid problems associated with a low power factor, practical AC-DC power converters typically employ a power factor correction (PFC) pre-regulator 302 between the output of the bridge rectifier 102 and the input of the DC-DC converter 106, as illustrated in FIG. 3. The PFC pre-regulator 302 functions to force the input current Iin to be more sinusoidal and in phase with the AC input voltage Vin, thereby increasing the power factor. Unfortunately, introduction of the PFC pre-regulator 302 lowers energy efficiency, increases parts count and manufacturing costs, and makes it difficult to package the LED bulb 300 in a small form factor. Moreover, the PFC pre-regulator 302 usually contains a boost converter that generates high voltages. These high voltages tend to stress the LED bulb's 300 parts, leading to reliability problems. The high voltages also pose safety concerns.
Yet another problem with existing LED bulbs relates to their inherent inability to be controlled by conventional dimmer switches. Many homes and offices have dimmer switches that were designed to control the dimming of incandescent bulbs. It would be desirable to be able to use those pre-installed dimmer switches to control the dimming of LED bulbs.
FIG. 4 is a circuit diagram showing a conventional dimmer switch 400. The dimmer switch 400 comprises a variable resistor 402, a capacitor 404, a DIAC (diode for alternating current) 406, and a TRIAC (triode for alternating current) 408. The TRIAC 408 is triggered when the voltage across the capacitor 404 exceeds the breakdown voltage of the DIAC 406. The voltage increases and decreases according to the cycling of the AC input voltage Vin, and triggering of the TRIAC 408 is delayed for each positive and negative half cycle depending on the RC delay presented by the variable resistor 402 and capacitor 404. Accordingly, the turn-on delay 512 results in a distorted dimming waveform with a lower average power, as illustrated in FIG. 5B. In angular terms, the turn on delay 512 is referred to in the art as the “firing angle” (180°-θ), where θ is known as the “conduction angle.” The ability to control the firing angle by adjusting the variable resistor 402 therefore provides the ability to control the average power delivered to the incandescent bulb 410 and, consequently, the dimming of the incandescent bulb 410.
The TRIAC dimmer switch 400 is suitable for controlling the dimming of incandescent bulbs. Unfortunately, it does not provide an acceptable solution for dimming existing LED bulbs, like the prior art LED bulbs 100 and 300 in FIGS. 1 and 3. Incandescent bulbs present a resistive load during all portions of the AC input waveform cycle. However, LEDs are nonlinear devices and draw significantly less current than do incandescent bulbs. At increased dimming (i.e., low light output levels) in particular, the current drawn by the LEDs of existing LED bulbs can be so small that the current drops below the holding current of the TRIAC 408. Under these conditions, the TRIAC 408 can retrigger or turn OFF, resulting in annoying LED flickering, or the LED bulb prematurely turning OFF before reaching the desired dimming level. The presence of the AC-DC power conversion circuitry between the AC power source and LEDs can also interfere with the ability of the TRIAC dimmer switch 400 to control the dimming of the LEDs.
Considering the foregoing drawbacks and limitations of existing LED bulbs, it would be desirable to have power conversion and control methods and apparatus for LED bulbs that are energy-efficient, inexpensive to manufacture, compact, safe to use, reliable, and provide the ability to control the dimming of LEDs of the LED bulb over a wide dimming range using conventional dimmer switches.